Summary:

Groins are structures often used in lowland river regulation. The main goal of their construction is protection of coast and narrowing of the river channel to increase water depth and prolongation of the navigation period. Analysing the influence of groins on hydrodynamics and sediment transport in rivers it is necessary to consider two cases: (1) non-submerged groins, when the groins top is above the water surface, and (2) submerged groins. In case of nonsubmerged groins the narrowing of the main channel is observed; it is accompanied by increase of water velocity leading to further deepening of the main channel. In the groin fields water flows much slower, supporting sediment deposition. In case of submerged groins water flows in the whole channel, but increase of resistance due to groins causes reduction of its capacity, being an important aspect in flooding periods. Water flow is always accompanied by sediment transport, but the existing knowledge in this respect is still insufficient to allow prediction of morphological changes with high accuracy. Groins are constructed since few centuries but not always the expected results are reached; it is not unique that they create new problems (deepening, sediment deposition). Recently laboratory experiments are carried out to solve simplified problems; less common field measurements are carried out. Tests on hydraulic models allow better understanding the mechanisms governing water and sediment movement in rivers trained by groins. They also support us with data necessary to verify mathematical models. The on-going developments in numerical modelling result in better representation of changeable hydro- and morpho-dynamics; however the results are still far from being able to reproduce all phenomena observed in nature.